1. Field of the Invention
This invention relates to an optical device in optical communication systems, more particularly, to a waveguide type optical divider for multimode optical fibers with graded refractive-index, which is used for optical communications, optical measurements and so forth.
2. Description of the Prior Art
An optical divider is an important device for dividing an input light signal into a plurality of optical signals or systems in the field of optical communications or optical measurements. As for the optical divider, there are several types of optical dividers such as a half mirror, a fused and extended optical fiber, a plane waveguide (hereinafter referred to briefly as waveguide), made of such a material as a polymer, glass and the like. Since a waveguide type optical divider has properties such that it is highly flexible in designing, easy to fabricate multibranch circuits and so forth, various investigations about such optical dividers have been executed in the past. Particularly, multimode optical fibers of graded-index type (hereinafter referred to briefly as "GI") are widely used in optical communication systems for a comparatively short distance such as in a local optical network, and it has been longed for providing an optical divider in good quality for the GI type optical fibers.
Principal indices for the properties of such optical dividers are optical loss and loss variance. It is known that a cross sectional shape of an optical waveguide can be formed substantially into a circle as in a glass waveguide provided by an ion migration method, however, in most cases, optical waveguides are of rectangular cross sections. When an optical fiber having circular sectional shape is connected to an optical waveguide having a rectangular cross section to provide an optical divider, the coupling loss due to a mismatching in shape (hereinafter referred to as shape loss) may occur, and the shape loss contributes greatly to the optical loss of the optical divider. Thus, in order to improve performance of an optical divider, it is important to reduce such shape loss.
On the other hand, for improving the loss variance of the multimode optical divider, there has been proposed a method in which a width of a branched optical waveguide at a central portion is made comparatively narrow relative to that of an end portion because of a fact that the intensity distribution of transmitting light in a main optical waveguide is strong at the middle (center) portion but weak at the end (peripheral) portion as described in Japanese Patent Laid-open No. 62-69205. However, in that publication, a method of designing a branching optical waveguide and the effect thereof are not disclosed.
It is therefore an object of the present invention to provide an optical divider having a low optical loss and an improved loss variance for use with optical fibers of GI type.
Firstly, a cause of occurring a shape loss, which is the main factor of the optical loss, will be described. Jointing conditions (jointing section) between optical fibers and wave guides are normally such that as shown in FIG. 1A (schematic cross sectional view at input side) and FIG. 1B (schematic cross sectional view at output side). Generally, a core width D of a main optical waveguide at the input side is greater than a core diameter K of an optical fiber. Accordingly, the optical loss in a direction of width when a light from an input optical fiber 7 enters a main optical waveguide 2 can be reduced theoretically to zero. However, as to a direction of thickness, the optical loss at portions indicated by hatching cannot be eliminated depending on a core thickness T of the optical waveguide. Theoretically, such optical loss in the direction of thickness can be reduced to zero by making the core thickness T of the optical waveguide core greater than the core diameter K of the optical fiber (T&gt;k), however, this results in the optical loss at the output side.
Accordingly, in order to fabricate an optical divider with low optical loss, the core thickness T of the optical waveguide must be selected and designed to be an optimum value, at first, for reducing the shape loss.
Secondly, at the output side, if a core width W of a branched optical waveguide is smaller than a dimension which is inscribed in a circle having the core diameter K of the optical fiber, an output light from the branched optical waveguide 3 will enter an optical fiber 7 and there will be no shape loss. However, if the core thickness T of the waveguide is set to a value which is very proximate to the core diameter K of the optical fiber (dotted lines in FIG. 1B), then the optical loss at the input side may be reduced, but if the width W of the branch optical waveguide is designed excessively small with respect to the core thickness T of the optical waveguide, there will be some difficulties in forming an optical circuit as well as fabricating the optical circuit.
In order to reduce the shape loss of an optical divider, optimization of the core thickness T of the optical waveguide and the width W of the branch optical waveguide with respect to the core diameter K of the optical fiber should be achieved at first, but this is not easy, heretofore, because the optimization has been carried out by way of trial and error as well as experiences.
If there is a difference in refractive-index between a central portion and a peripheral portion of the optical waveguide, a power at the terminal end of the optical waveguide may vary in accordance with a value of the refractive-index. In particular, the light being transmitted through a sufficiently long optical waveguide in a stationary mode has high intensity at the central portion but low at the peripheral portion of the output end of the optical waveguide and cannot be branched uniformly. However, after the inventors have studied for an improvement of the optical divider for use with the optical fibers, it has been found that the power distribution at the terminal end of the branched optical waveguide varies depending upon a shape, a length and a width of the main optical waveguide, and the optical divider having an improved loss variance can be fabricated by optimizing those conditions.
It is therefore an object of the present invention to provide an improvement in an optical divider for use with a multimode optical fiber of GI type which is low in optical loss and superior in loss variance.